1 Paraba Federal Institute of Education,

Atlantic forest, conservation systems, soil

organic carbon, soil aggregates,sustainability

Accepted: 14.04.2014

griculture is a very representative activity in Brazil, one of the main

economic resources of the country. However, the improper management ofagricultural areas has negative effects on key soil properties such as soil organicmatter. The aim of this research was to identify changes in soil organic carbonand its distribution in soil aggregates, depending on the transformation ofnative vegetation areas on pasture lands and croplands. The study wasconducted in Pitimbu, State of Paraba, Brazil. Three representativemanagement systems were selected in a Ultisol: bean crop under conventionaltillage (TIL), planted pasture (PAS) and native forest in Atlantic Forest biome(NAT). In each plot, samples of soils were collected at depths of 0-20 and 20-40cm. Total organic carbon (TOC) was determined in the soil fraction <2 mm,according to Walkley & Black (1934). Aggregates between 8.00 and 4.76 mmwere subjected to a 15 minutes wet sieving with vertical oscillation (Yoder,1936), and fractions of aggregates were obtained in the following classes: (A)4.76 to 2.00 mm, (B) 2.00 to 1.00 mm , (C) 1.00 to 0.50 mm, (D) 0. 50 to 0.25 mm,(E) < 0.25 mm. The TOC of each class of soil aggregates were determined by thesame method used in the total mass of soil. The results were subjected toANOVA and mean comparison by Tukey test at 5%. There was no significantdifference in TOC content of the total soil mass between TIL, PAS and NAT, andno significant effect of the sampling depth. However, TOC content in NAT (3.71mg kg-1) was higher than TIL (1.85 mg kg-1) and PAS (2.66 mg kg-1) along thesampled profile in A aggregate class (P0.0006). NAT also surpassed TIL in bothB aggregate class (P 0.0445, only in the layer of 20-40 cm) and C class(P0.0287). Plant diversity observed in the forest, the dense layer of leaf litterand the abundant presence of the root system of native species make NAT veryprotective of the carbon contained in the aggregates, especially in largeraggregates. The introduction of annual cropping and pasture in forest nativevegetation areas in similar conditions to this research alters the distribution oforganic carbon in soil aggregates. Studies on the sustainability of environmentsmust consider not only the content of TOC in total mass of the soil, but alsowithin aggregates.

INTRODUCTIONAgriculture is a very representative activity inBrazil, one of the main economic resources o f thecountry, currently representing 5.7% of GrossDomestic Product (GDP). During the first quarterof 2014, GDP in the Agriculture sector grew by 3.6 %

compared to the fourth quarter of 2013 with 4.8 %

cumulative growth over the last four quarters,indicating the strength and importance of the sectorfor the Brazilian economy (IBGE, 2014). Accordingto the Ministry of Agrarian Development of Brazil,family farms grew 52 % over the last ten years, and

Gomes et al.

now accounts for 4.3 million production units (84% of

farms in the country) and 33% of GDP in theAgriculture sector, employing 74% of the labor forcefield (BRASIL, 2013). Although rates of deforestationare suffering reductions in Brazil from the 2004-2005period, deforestation is still a worrying form of landuse change in the country, having contributed in 2008to a loss of 40% of soil carbon to atmosphere, causingover recent decades accelerated reductions in thearea of major biomes such as the Amazon, the Cerradoand the Atlantic Forest (World Bank, 2010). In thiscontext, soil quality indicators are useful to monitorearly changes in key soil properties (Islam & Weil,2000) such as soil organic matter (SOM) andaggregate stability indices (Karlen & Stott, 1994).In the transformation of areas of native vegetationon agricultural land, increased rates of SOM oxidationcauses reductions on aggregate stability, particularlyin macroaggregates, which are very sensitive tomanagement, increasing in number in the case ofareas with grasses and reducing when the soil isdisturbed by intensive cultivation (Oades, 1984).Several conditions of soil quality reductions have beenobserved in Brazil because of intensive soilmanagement in agricultural areas, leading to negativeeffects on SOM, aggregation and microbial biomass (D'Andrea et al 2002a, b; Wendling et al, 2005; CostaJunior et al, 2012). Passos et al. (2007) noted a markedreduction in the organic carbon of smaller aggregates(<0.106 mm) compared to the larger size aggregates,due to the fact that larger body reserves are mainlyrelated to clay so that there is an increase in the rate ofmineralization with decreasing particle (clay> silt>sand). Maia et al. (2009) argue that factors such asspatial variability, climate and soil types, in addition tomanagement history, are seen as the main factorsresponsible for differences in the soil organic Ccontent, indicating that few studies have beendeveloped in soils under pasture taking intoconsideration soil aggregation and the dynamics of C.Moreover, the gradual reduction of native ecosystemsdue to land use changes contributes to the reductionof biodiversity, loss of natural habitats and significantchanges in regional water balance, by the expansionof agricultural areas or by the pressure of disorderedurban growth. The aim of this research was to identifychanges in soil organic C and its distribution in soilaggregates, depending on the transformation ofnative vegetation areas on pasture lands andcroplands.

MATERIAL and METHODS

The study was conducted in the municipality ofPitimbu, State of Paraba, Brazil, located in the middleregion of Zona da Mata Paraibana, micro-region of theSouth Coast (Litoral Sul) and metropolitan area of JooPessoa. The climate is As' according to KppenClimate Classification (tropical climate with rainseason on fall-winter), with average annual rainfall of1,634 mm mm yr-1 and an average annual temperaturebetween 25 and 29C. The study area belongs to thegeomorphological unit of the Coastal Tablelands(Tabuleiros Costeiros), geological formation GrupoBarreiras Indiviso (BRASIL, 1981). The study area islocated on a Ultisol (Argissolo Vermelho Amarelo,Brazilian soil classification), in the agriculturalsettlement of APASA, occupied by small farmers withagroecological approach (Lima, 2008). For samplingwe selected three representative managementsystems: bean crop under conventional tillage (TIL),planted pasture (PAS) and native forest in AtlanticForest Biome (NAT). The average contents of clay, siltand sand were 182 g kg-1, 4 g kg-1 and 814 g kg-1 formanaged areas (TIL and PAS) and 269 g kg-1, 46 g kg-1and 685 g kg-1 for the NAT area. In each selectedmanagement system, samples of soils were collectedat depths of 0-20 and 20-40 cm. Total organic carbon(TOC) was determined in the soil fraction <2 mm,according to Walkley & Black (1934). Aggregatesbetween 8.00 and 4.76 mm were subjected to a 15minutes wet sieving with vertical oscillation (Yoder,1936), and fractions of aggregates were obtained inthe following classes: (A) 4.76 to 2.00 mm, (B) 2.00 to1.00 mm , (C) 1.00 to 0.50 mm, (D) 0, 50 to 0.25 mm, (E)< 0.25 mm. The TOC of each class of soil aggregateswere determined by the same method used in thetotal mass of soil. The results were subjected toANOVA and mean comparison by Tukey test at 5%.RESULTS and DISCUSSIONThe analysis of variance (ANOVA) of total organiccarbon (TOC) of the total mass of the soil revealed nosignificant difference between NAT, TIL and PAS (P 0.0907) systems. The average TOC in NAT, TIL and PASsystems were 3.16 g kg-1, 2.97 g kg-1 and 2.41 g kg-1,respectively. Also no significant differences wereobserved in average TOC of the total mass of soilbetween the two sampling depths (P 0.1794), withmean values of 3.00 g kg-1 for the 0-20 cm layer and 2,69 g kg-1 for the 20-40 cm layer. In contrast to levels ofTOC in the total mass of soil, ANOVA of total organiccarbon TOC in the different fractions of aggregates

Organic carbon in soil aggregate fractions under native vegetation and agricultural use in Pitimbu, Paraiba, Brazil

revealed significant differences among the systems

studied, mainly in fractions of larger aggregates(Figure 1).The TOC content in the fraction of aggregates>2.00 mm was significantly higher in NAT system(3.71 g kg-1) compared to more intensive systems PAS(2.66 g kg-1) and TIL (1.85 dag kg-1). The trend wasrepeated in the fraction of aggregates with sizebetween 2-1 mm, although there was a significantinteraction of the management systems andthe sampling depth (P0.0421). In this case, there was

effect of soil management on soil carbon only in the

20-40 cm layer, where TOC in aggregates of 2-1 mmwas higher in NAT system (2.20 g kg-1) and lowest inTIL (1.12 g kg-1). Significant differences in TOCconcentrations were also found in the fraction ofaggregates between 1-0.5 mm, with higher carboncontent in the NAT (1.74 g kg-1) compared to the TILsystem (0.97 g kg-1), in the average of the two depths.In the fractions of aggregates between 0.5-0.25 mmand <0.25 mm, no significant differences were foundin the TOC.

The results clearly indicate differences between

the management systems in relation to TOC contentin classes of aggregates larger than 0.50 mm. Largeraggregates stored more organic carbon in NAT than inmore intensive systems, especially TIL, being soilaggregates TOC a good indicator for the preservationof forest environments. Taking into account thehierarchical classification of aggregates proposed byTisdall & Oades (1982), in which the diameter of250m separates the microaggregates ofmacroaggregates (the latter formed by the union ofsmaller aggregates), one sees easily that themacroaggregates of the studied soil are less resistantto carbon loss resulting from the action of cultivation.In the present work, the results are consistent withthose found by Costa Junior et al (2012). Theseauthors observed in a Brazilian Oxisol with claycontents ranging from 541 g kg-1 and 641 g kg-1 (in thetop 0-20 cm), that aggregates larger than 0.25 mmmay be more sensitive to land use change, offeringless protection of soil organic carbon, especially in thelayer of 0-5 cm of soils under native vegetationconverted into cultivated and pasture systems. In thepresent work, less TOC in the larger aggregates canreduce its stability, which should be furtherinvestigated. In this sense Bastos et al. (2005) arguethat the polysaccharides are the active agentsinvolved in the stability of aggregates that are rapidlydecomposed, being directly linked to the stability ofsoil macroaggregates. Other agents such as fungalhyphae and roots that take longer to decomposeremain in soil for weeks and even months and are alsoassociated with macroaggregates.On soil, management systems that add organicmatter mainly through plant residues can increase thecontent of TOC, contributing to the maintenance ofagricultural soil sustainability and reduction of CO2emissions to the atmosphere (Loss, 2011). Moreover,the absence of soil disturbance into native forest oreven in conservation farming systems such as no-tillsystems may contribute to increasing organic carbonwithin aggregates (Costa Junior et al., 2012). Thus, thedynamics with which the aggregates are renewed canbe a major factor that controls the relationshipbetween the occlusion of organic matter inaggregated to control its decomposition dynamics(Rilling & Mummey, 2006).In this work, the soil aggregates in bean crop areahad low levels of carbon in all size fractions, beingsignificantly lower than TOC content of native forest inthe larger size aggregates. It was observed in thefield that the diversity of vegetation in native forest is

abundant, and the root system of native species

makes the forest a very protective system of thecarbon contained in the aggregates, especially in thelarger ones. In the present study, the pasture wasshown as an intermediate system between the forestand the conventional cultivation of beans, especiallyin fractions larger than 0.5 mm aggregates. Thetransformation of areas of native forest to pasture orannual crops under similar conditions to those in thisstudy alters the distribution of organic carbon in soilaggregates, although no significant differences wereobserved with respect to values of total soil mass.Studies on the sustainability of environments mustconsider not only the content of TOC in the total massin the soil, but also its distribution within theaggregates. Further studies can be conducted toevaluate the state of soil aggregation through indexesaggregate stability in the systems under study, as wellas the evaluation of the microbial biomass of the soil,the rate of soil respiration and decomposition rate ofsoil organic matter. Authors have reported changes inthe microbial components of soil in the surface layersof both sandy and clay soils in Brazil after thetransformation of areas with native vegetation intoagroecosystems (D'Andrea et al, 2002a; Frazo et al2010).Concerned with issues relating to the transfer ofcarbon from the terrestrial compartment to theatmosphere, the Brazilian Ministry of Agriculture,Livestock and Supply (MAPA) produced in 2012 theSectoral Plan for Mitigation and Adaptation to ClimateChange for the consolidation of an low carbonagriculture economy, also called Plan ABC (LowCarbon Emission Agriculture). This plan aims to reducegreenhouse gases emissions in agriculture inaccordance with the National Policy on ClimateChange through the adoption of sustainableproduction technologies to meet the commitmentsmade by the agricultural sector in Brazil (BRASIL,2012). Among the commitments are: a) encouragingthe introduction of planted forests (target areaincrease of approximately 3 million ha); b) therecovery of degraded pastures (target area increase ofapproximately 15 million ha) and c) crop-livestockforest (with goals of increased area of about 4 millionhectares). Such policies in the agricultural sectorshould contribute significantly to improve thephysical state of the soil, maintain or increase carbonstocks in terrestrial compartment, and reduce its ossesto the atmosphere, with direct impacts related to

Organic carbon in soil aggregate fractions under native vegetation and agricultural use in Pitimbu, Paraiba, Brazil

encouraging the adoption of practices of conservation

agriculture.

must consider not only the content of TOC in total

mass of the soil, but also within aggregates.

CONCLUSIONThe introduction of annual cropping and pasturein forest native vegetation areas in similar conditionsin this research alters the distribution of organiccarbon in soil aggregates, with a significant reductionof the levels of TOC in aggregate classes greater than0.5 mm. Studies on the sustainability of environments

ACKNOWLEDGEMENTSThe authors thank IFPB (Federal Institute ofEducation, Science and Technology) Joao Pessoacampus for the financial and institutional support(research grant to the corresponding author) and IFPBSousa campus for the support in the soil laboratoryanalyzes.